While conventional brush-commutated DC motors may have numerous advantageous characteristics such as convenience of changing operational speeds and direction of rotation, it is believed that there may be disadvantages, such as brush wear, electrical noise or RF interference caused by sparking between the brushes and the segmented commutator, that may limit the applicability of such brush-commutated DC motors in some fields such as the domestic appliance field. Electronically commutated motors, such as brushless DC motors and permanent magnet motors with electronic commutation, have now been developed and generally are believed to have the above discussed advantageous characteristics of the brush-commutated DC motors without many of the disadvantages thereof while also having other important advantages. Such electronically commutated motors are disclosed in the David M. Erdman U.S. Pat. Nos. 4,005,347 and 4,169,990 and Floyd H. Wright U.S. Pat. No. 4,162,435, all of which are commonly assigned with the present application. These electronically commutated motors may be advantageously employed in many different fields or motor applications among which are domestic appliances, e.g., automatic washing or laundry machines such as disclosed in commonly assigned, co-pending U.S. patent applications Ser. No. 412,421 filed Aug. 27, 1982, now U.S. Pat. No. 4,449,079; Ser. No. 367,951 filed Apr. 13, 1982; Ser. No. 400,319 filed July 21, 1982; Ser. No. 191,056 filed Sept. 25, 1980, now U.S. Pat. No. 4,459,519; Ser. No. 141,268 filed Apr. 17, 1980, now U.S. Pat. No. 4,390,826; Ser. No. 077,784 filed Sept. 21, 1979, now U.S. Pat. No. 4,327,302; and Ser. No. 463,147 filed Feb. 2, 1983.
Coassigned U.S. Pat. No. 4,250,544, Combination Microprocessor and Discrete Element Control System for a Clock Rate Controlled Electronically Commutated Motor issued Feb. 10, 1981, to R. P. Alley is incorporated herein by reference.
Application Ser. No. 463,147 to David M. Erdman is also related to the present application and is incorporated herein by reference.
Laundry machines as disclosed in the above patents and applications are believed to have many significant advantages over the prior art laundry machines which employ various types of transmissions and mechanisms to convert rotary motion into oscillatory motion to selectively actuate the machine in its agitation or washing mode and in its spin extraction mode. Such prior art laundry machines are believed to be more costly and/or complicated to manufacture, consume more energy, and require more servicing. Laundry machines with electronically commutated motors require no mechanical means, other than mere speed reducing means, to effect oscillatory action of the agitator, and in some applications, it is believed that the spin basket might be directly driven by such a motor. While the past control systems, such as those disclosed in the aforementioned coassigned applications for instance, undoubtedly illustrated many features, it is believed that the control systems for electronically commutated motors in general, and for such motors utilized in laundry machines, could be improved. In some of the past control systems, the position of the rotatable assembly (i.e., the rotor) of the electronically commutated motor was located by sensing the back emf of one of the winding stages on the stationary assembly (i.e., the stator) thereof. More particularly the back emf of an unenergized winding stage was sensed and integrated to determine rotor position. However, the voltage measured at the terminal of an unenergized winding has several components other than that induced by the rotation of the rotor. Immediately after a winding is commutated off, the voltage at the terminal of the now unenergized winding crosses zero. Thereafter the voltage is of the same polarity as the anticipated back emf at the end of the commutation period, but it is not due to back emf. Rather, current which had been in the winding while it was energized induces this field collapse voltage, so it is not an accurate measure of rotor position. This field collapse voltage may last for several electrical degrees, but its actual duration is highly motor and load dependent. Some of the previous applications disclose locking out the integrator for a predetermined number of electrical degrees of rotation, e.g., 20.degree., to prevent the current-induced voltage from being integrated. This has been done successfully by basing the measurement of the lockout interval upon the immediately previous interval between commutations. However, during rapid speed changes, it is believed that this approach can result in lockout times which are longer than 60 electrical degrees or less than the commutation current interval, both of which may result in a loss of position sensing and in less than desirable motor operation. A system which ignores these commutation current-induced voltages and avoids the problems which may result from locking out the integrator for a supposedly constant number of electrical degrees would be desirable.
The speed of an electrically commutated motor is directly correlated to the average voltage applied to the windings, which in turn is determined by the unregulated DC voltage applied to the motor windings and the duty cycle of the pulse width modulation used to apply the voltage. The duty cycle is in turn a function of the total time the windings are energized each cycle divided by the length of each cycle. The windings are typically energized until some function of the supply voltage, such as the integral, equals some preset reference selected to give the desired average voltage. When this reference is reached, the winding drive is shut off. The length of the cycle has been separately determined, usually by a separate clock circuit. In economical circuits, both the generation of the function of applied voltage and the measurement of the cycle length are believed to introduce error into the average voltage applied. Use of inexpensive low precision capacitors for each function in some cases may result in significant error in average voltage applied, and hence in motor speed. Conversely, higher precision capacitors are undesirably expensive for use in a control system which is to be widely and economically used. Thus, it would be desirable to have a control system which uses relatively low precision components yet which accurately controls average applied voltage and motor speed.